The communications between a mobile terminal and a serving cell, such as the uplink (UL) communications from the mobile terminal to the serving cell, may sometimes affect resource utilization, throughput, latency and coverage. To enhance UL improvements, the ongoing evolution of wireless communications systems such as, for example the enhanced dedicated channel (E-DCH) in CELL_FACH state feature was introduced into wireless standard specifications, such as the third generation partnership project (3GGP) Release 8 specifications.
Mobile terminals operating in a CELL_FACH mode may use a contention based E-DCH channel for UL transmission rather than a traditional random access channel (RACH). The contention-based E-DCH channel allows for mobile terminals to transfer signaling and data at significantly higher data rates and for longer durations, which reduces transfer and state transition delays.
In order to limit errors due to fading and interference on the uplink from a mobile terminal (also known as user equipment) to the network, data may be divided into blocks with the bits within a block being encoded and interleaved. The size of a block is related to a transmission time interval (TTI) as each block must be transmitted within the TTI. Upon receipt of a block, a network entity may de-interleave and decode the bits and may then estimate the bit error rate as a result of the transmission of the block within the TTI. The TTI is therefore the shortest period over which the network entity can estimate the bit error rate. In networks that utilize link adaptation techniques based on the estimated bit error rate, the shortest interval between reports of the estimated performance, that is, between reports of the bit error rate, is a single TTI. Thus, the length of the TTI generally limits a network's responsiveness in adapting to changed conditions on the uplink. In order to adapt more quickly to the changing conditions on an uplink, shorter TTIs may be desired. However, the desire for short TTIs may be offset by the increased efficiencies in error correction and compression techniques and the increased benefits from interleaving that are provided by longer TTIs.
Various TTIs have been defined including a 10 ms TTI and a 2 ms TTI. For example, support for concurrent deployment of TTI settings, (e.g., 2 ms and 10 ms), will be allowed for the common E-DCH in the CELL_FACH state. In this regard, 3GGP Release 11 (Rel-11) supports concurrent deployment of 2 ms and 10 ms TTI settings in a cell. A single TTI setting, which may be determined and broadcast by a communications network, may be used by mobile terminals accessing the E-DCH in the CELL_FACH state within a particular cell.
The 2 ms TTI allows for a quicker adaptation to uplink conditions, thereby allowing the transmission of data blocks to be quickly scheduled while the uplink is temporarily enjoying improved conditions. As a result, a 2 ms TTI may allow data to be transmitted over an uplink having better than average conditions such that the bit rate transmitted via the uplink may be higher than that allowed by average conditions, thereby correspondingly increasing the network capacity. However, a smaller TTI, such as a 2 ms TTI, may only be workable in an instance in which the mobile terminal or other user equipment is located within a central portion of a coverage area since a mobile terminal or other user equipment near the edge of the coverage area may be unable to successfully transmit data in accordance with the smaller TTI.
Thus, in an instance in which a smaller TTI was initially selected, such as in an instance in which the mobile terminal was within a central region of the coverage area at the time that the smaller TTI was selected, the mobile terminal may need to switch to a longer TTI in an instance in which the mobile terminal moves so as to be closer to the edge of the coverage area. Additionally, the initial selection of the TTI may, in some instances, be based upon an inaccurate measurement such that the initial selection of a smaller TTI may be incorrect, thereby creating an immediate desire to switch to a longer TTI.